1. Field of Invention
The invention is related to electrical oscillations and in particular to crystal oscillators where there are multiple modes of resonance in the crystal and it is desired to establish oscillations on a particular mode of the crystal.
2. Description of Prior Art
A quartz crystal oscillator consists of a crystal resonator and a sustaining circuit. The quarts crystal itself has various modes of resonance such as the fundamental thickness sheer mode, overtone thickness sheer modes, extensional modes, flexural modes, etc. In many cases one of the modes is significantly more active than the others and oscillations commence on the frequency of that mode. It is often possible to design the sustaining circuit to favor a particular mode of oscillation, e.g. the third overtone rather than the fundamental mode and produce oscillations on that frequency. In the case of a stress compensated (SC) quartz crystal, selection of a C-mode or B-mode is also possible. The B and C modes are close in frequency, within 10%, and fairly sharp frequency selection circuits are necessary in the sustaining circuit to select a particular mode. The use of sharp frequency selective elements in the sustaining circuit generally results in a degradation of the frequency stability of the quartz resonator because of variations in the selective elements such as inductors that change in value with temperature, with time or exhibit retrace characteristics.
If the strengths of the modes in the crystal are not too different, once oscillation has been established on a particular mode and saturation has occurred on that mode, the highly frequency selective elements can be switched out of the oscillator to enhance the stability. This is discussed in U.S. Pat. No. 654,550 in the case of sequenced multi-mode oscillators. The mode control networks may require precise tuning such as with a varactor and it may in some cases be necessary to adjust the tuning voltage for the particular ambient temperature. This is an obvious complication. The electronic switches used to remove the mode control network may also be imperfect resulting in some residual frequency pulling effects.
The present invention provides an alternate means of steering the oscillator to the correct mode by injecting a signal with energy close to the desired mode into the resonator during the build up of oscillations. Normally when an oscillator is turned on, all mode for which the loop gain is greater that unity and the phase shift is 360 degrees, begin to build up simultaneously from the residual noise level of the sustaining circuit. The mode reaching the saturation amplitude first then causes a relative reduction in gain at frequencies different from the saturating mode. This causes the other modes to die out. Injection of a signal significantly higher than the noise level of the sustaining circuit, if it is close to a specific mode, may often be sufficient to insure that the favored mode will survive. The procedure may also significantly decrease the time required for oscillations to build up on the selected mode. After saturation has occurred, the desired mode may remain locked to the injection signal if it is a sine wave until it is remove if it is close enough to the natural resonance of the oscillator. If such locking does occur it is only momentary since the injection signal is removed after saturation has occurred. Injection locking of a oscillator under steady state conditions is well known in the art and is a method of precisely controlling the frequency of an oscillator. A harmonic or sub-harmonic is often used for locking in this case. The injection signal must, of course, be present continuously for such frequency control to occur, and the two frequencies must be precisely the same of precisely related harmonically.
In the present invention the injection mode control signal is present only during the build up of oscillation, and it is in general not necessary for the injection signal to be close enough in frequency to the natural resonance of the oscillator so that locking would occur if it remained present after saturation. It is only necessary that the injected signal create enough energy in the vacinity of the desired mode so that the desired mode builds up faster that the other modes that are to be discriminated against. The injected signal may be a sine wave or it may be band limited noise. The use of band limited noise may be advantageous since it avoids the necessity to position the center frequency of the injection as precisely as a sine wave to be effective.
The mode steering injection signal can easily be generated by the use of a direct digital frequency synthesizer (DDS) in connection with a second crystal oscillator that provides the clock for the DDS. The power spectrum of a DDS can also easily be broadened by applying phase or frequency modulation. Even in the case of a sine wave, the injection mode control signal need not be precisely positioned and the second oscillator can be of lesser quality than that of the injection steered oscillator. In one application of the invention a second crystal oscillator is already present in the application and that oscillator is used as the clocking signal for the DDS. It is not necessary for the DDS to provide a spectrally pure signal as before mentioned since the only requirement is to provide increased noise in the vicinity of the desired mode of resonance. This may result in a simplified DDS so that it consist essentially of an adder and a phase accumulator. If the DDS is modulated with bandlimited noise, the allowable separation between the mode steering frequency and the desired mode may be advantageously increased.
It is worth noting that the use of an injection signal to enhance the power spectral density in the vacinity of a particular mode to reduce the build up time for that mode during start up is fundamentally different from the case where a locking signal is injected into an oscillating circuit resulting in a phase shift of the feedback signal to cause frequency locking. Such a signal must be very close to the natural frequency of the oscillator and it must remain present during operation of the oscillator. It must also be large enough to maintain an influence on the phase after the amplitude has become large in steady state. The requirement on the mode steering signal is only that it must be significantly larger than the residual noise level of the sustaining circuit in the vicinity of the desired mode.